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Hydraulics Failure

Most helicopters use hydraulics to reduce the force needed to move the controls. Usually there is still a mechanical connection. In some helicopters there might not be a mechanical connection, or the control forces may simply be too high for the pilot to overcome with muscle. Generally those sorts of helicopters will have redundant hydraulic systems.

Both the Bell JetRanger/LongRanger and the Robinson R44 have boosted controls with mechanical backup. There are multiple ways such hydraulic systems can fail, but basically you can either have a failure where the entire system stops working, or a failure where a part of the system stops working.

Total Hydraulic Failure

Of the two failure modes, this is probably the easier to deal with. When the failure occurs, the pilot will notice that the control boost is gone, and he will have to fly the aircraft using much more force to move the controls. In the case of the cyclic and collective, he will also usually notice that they are connected, i.e. when he moves one control, the feedback from the rotor system will cause the corresponding control to also try to move. This effect is small and can easilly be overcome.

In a small helicopter such as the Bell or the Robiinson, the stick forces are low enough that the pilot can fly the aircraft with little difficulty. If the pilot has to fly for a significant amount of time before landing, he will probably be fatiqued and it may be difficult to avoid overcontrolling the helicopter while attempting to hover. The aircraft manufacturers generally recommend a running landing instead of attempting to hover in order to avoid this.

Partial Hydraulic Failure

A more difficult failure may be when one hydraulic servo fails, but the others continue to work. This means that his controls are boosted in some parts of their movement, but not in others. Such a failure could easilly result in an aircraft that is not flyable by the average pilot, so Bell and Robinson both provide a switch to allow the pilot to disable the hydraulic system. When the pilot activates the switch, the entire hydraulic system is defeated meaning that the pilot still has to contend with a total hydraulic failure, but at least all the stick forces are equally high, and the average pilot should be able to fly the aircraft with little trouble.

Both the Bell and Robinson hydraulic override systems are "failsafe". That is, it takes an electrical circuit to hold the hydraulic system in the override condition. If the electical system should fail, the hydraulics will continue to be boosted regardless of the position of the hydraulic override switch. The prevents loss of the aircraft electrical system from also causing a hydraulic systems failure.

Hardover Failure

Another failure mode of a hydraulically boosted control system is that one of the servos could be driven to one extreme position or the other by a failure within the servo. This would result in an unflyable aircraft. The pilot can recover from such a failure by turning off the hydraulic system, assuming that the hardover failure did not cause him to lose control of the aircraft.
Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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